function arr2Str(arr:(number|string)[],str?:string){
  if(str){
    return arr.join(str);
  }else{
  return arr.join('!!');
  }
  }

  console.log(arr2Str([1,2,3],'-'));
  console.log(arr2Str([4,5],'&'));
  console.log(arr2Str(['9','8']));
  
